Design of reinforced concrete bridge columns for strength and ductility.

Abstract

Methods for the design of reinforced concrete bridge columns and piers for strength and ductility are considered. The investigations cover the following areas. An experimental investigation of the influence of reinforcing steel grade and amount of confining steel on the stress-strain behaviour of confined concrete is presented. The results are discussed and compared with theoretical models. Special attention is given to the possibility of fracture of the confining steel. An extensive experimental investigation into the ductile performance of a range of reinforced concrete columns is presented. The columns were subjected to constant axial load and cyclic lateral displacements. The test units included four square columns with the lateral load applied in the direction of a cross section diagonal, six circular hollow columns with different wall thickness to diameter ratios, and four columns with transverse reinforcement from Grade 380 steel. The available strength and ductility of the columns is discussed and compared with the performance of columns tested previously at the University of Canterbury, and with theoretical predictions using monotonic as well as cyclic moment-curvature analyses. The main variables for the solid columns were the influence of biaxial bending, the use of Grade 380 transverse steel for confinement, and the spacing between transverse bars along the column axis. The circular hollow columns were unconfined on the inside face of the tube wall, and the main variables were the influence of the axial load level and the wall thickness. The implications of the column test results, including the results of other investigations, for the design of reinforced concrete columns for strength and ductility are discussed and, where appropriate, used to calibrate theoretical models. In particular, the influence of cyclic loading on the strength deterioration of reinforced concrete columns with high axial loads is emphasized. More realistic definitions of the ideal flexural strength, of the flexural overstrength and of the yield curvature are suggested and, together with a set of criteria for the ultimate limit state, used to establish design charts for the available strength and ductility of reinforced concrete columns. A cyclic moment-curvature analysis was used for this purpose, incorporating cyclic stress-strain models for the concrete and for the steel, thus taking into account the cyclic strength deterioration observed for columns with high axial loads. Finally, a rational step-by-step design procedure is presented that will make less complex the task of considering the great number of variables involved in the seismic design of reinforced concrete columns for both strength and ductility.

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